The present invention relates to gas freezer-sublimers such as are employed in gaseous diffusion plants for enriching uranium in its U-235 content. Such freezer-sublimers are employed for the temporary storage of large gas volumes in their solid state to reduce storage space requirements.
The gaseous diffusion process for uranium enrichment is presently the most economical for the separation of the isotopes of uranium. The process is based on the fact that in a mixture of two gases of different molecular weight, molecules of the lighter gas will on the average be travelling at higher velocities than those of the heavier gas. If there is a porous barrier with holes just large enough to permit passage of the individual molecules but without permitting bulk flow of the gas as a whole, the probability of a gas molecule passing through the barrier will be directly proportional to its velocity. From kinetic theory it can be shown that the velocity of a gas molecule is inversely proportional to the square root of its molecular weight, so that the efficiency of gaseous diffusion will depend on the ratio of the square roots of the molecular weights of the two gases present. The two isotopes in uranium hexafluoride (UF.sub.6) to be separated are U-235F.sub.6 and U-238F.sub.6 and their molecular weight is 349 and 352, respectively. The efficiency of separation depends on the quantity EQU .sqroot.352/349 = 1.0043
Since this number is close to one, the separation is very small in any one step of the process. Accordingly, a large number of separation or diffusion stages must be combined in a so-called cascade.
In each diffusion stage UF.sub.6 gas is compressed, passed through a cooler to remove the heat of compression, and then admitted to a vessel containing the porous barrier. About half the gas entering the vessel diffuses through the barrier and passes to the next higher stage. This diffused gas contains a slightly higher concentration of the U-235 isotope. The undiffused gas is slightly depleted in the U-235 isotope, and passes to the next lower stage. More than a thousand individual stages are required to bring about the necessary overall change in composition in which the uranium is enriched to 2 to 4 percent U-235.
Gaseous diffusion plants of this type require substantial UF.sub.6 storage capacities. In the past, UF.sub.6 was stored in its gaseous form, taking up great amounts of storage space, or solidified in a freezer-sublimer on suitable surfaces so that a relatively large amount of UF.sub.6 can be conveniently stored.
Prior art UF.sub.6 freezer-sublimers principally comprise gas storage tanks in which finned heat exchanger tubes were mounted. The heat exchanger required means for pumping the freon coolant through the system in at least one direction and multiple valving to enable the reversal of the freon flow and the cooling water flow. These features made prior art freezer-sublimer systems costly and cumbersome to operate.